OCTOBEB 18, 1907] 



SCIENCE 



513 



end of the swing. When the pendulum is re- 

 leased from this, it swings back with little 

 assistance to the starting point and makes all 

 necessary adjustments automatically. On the 



back of the lower bob is an index point which 

 runs at the upper edge of the scale and serves 

 as a spark point. 



The record is made upon a smoked paper 

 which is seen through the slit above the scale. 

 This paper is stretched upon two rollers ; it 

 also rests upon an insulated metal plate which 

 serves as an electrode and keeps the paper 

 straight and smooth back of the scale. Back 

 of this plate is a third roller by means of 

 which the tension of the paper may be ad- 

 justed. The paper support is built on a 

 carriage so that it may be removed and re- 

 placed without disturbing the rest of the ap- 

 paratus. In preparing the paper this carriage 

 is removed and the paper is smoked as on an 

 ordinary kymograph drum. As a complete 

 record consists in a single spark which may 

 be recorded at once, several hundred records 

 may be made with one preparation of the 

 paper. The paper is moved, as needed, by a 

 thumb screw at one end of the upper roller. 



In reaction experiments the stimulus is 



given automatically by the apparatus when 

 the pendulum indicator passes the zero point 

 on the scale. A double rocking lever at this 

 point makes one circuit and breaks another, 

 either of which may be used in giving the 

 stimulus. These contacts are adjustable 

 platinum and mercury contacts and their ad- 

 justment may be verified by direct sight. The 

 closing or opening of the circuit is soundless, 

 and the stopping of the lever in a soft rubber 

 clutch makes no sound that can be heard a 

 few feet away. 



The reaction, or termination of the interval 

 to be measured, is indicated by a spark on 

 the sensitive paper at the edge of the scale. 

 The spark is produced by interrupting the 

 primary circuit of an ordinary induction coil. 

 One secondary terminal is connected with the 

 insulated plate on which the paper rests and 

 the other is connected with the body of the 

 apparatus. The point of the pendulum in- 

 dicator is the nearest metal to the plate; 

 therefore the spark flies from this point, 

 through the sensitive paper, to the plate. 



The scale is graduated empirically by the 

 most reliable graphic method into hundredths 

 of a second, and each unit is divided into 

 halves. The average space of one unit is 5 

 mm. on the arc of the scale. With this ad- 

 justment the scale covers 0.80 sec. and the 

 records are read in half-hundredths with ease 

 and accuracy. This division is the most con- 

 venient and appropriate to use in reaction 

 experiments. The variation in the movement 

 of the pendulum is negligible because the pen- 

 dulum is carefully constructed and balanced 

 and moves without friction. The variation in 

 the make contact is also negligible because the 

 platinum terminal moves much faster than 

 the pendulum indicator. The spark tends to 

 take the shortest course between the point and 

 the plate, but it may be deflected. The 

 maximum distance between the spark point 

 and the paper is 1 mn>>. The maximum de- 

 flection of the spark may be estimated to be 

 about 45°. That amount of deflection is not 

 liable to occur for the maximum distance, but 

 if it did the maximum variation would be 

 ± 1 mm. on the scale, which is equal, on the 



